The general objectives of this project are (1) to investigate radiation-induced changes in the immature (early postnatal) spinal cords, (2) to examine reparative and regenerative capacities of the immature nervous system and (3) to study normal postnatal development of the spinal cord. The experimental animal is the rat because the developmental status of the rat at birth makes it possible to study the immature nervous system ex utero. Two areas will be concentrated upon during this phase of the project: (1) the effects of age and of x-ray on the occurrence of the reactive (non-neuronal, non-vascular) cells that are observed in the central nervous system following injury to a peripheral nerve, and (2) the development of intramedullary Schwann cells and other histopathologic changes that occur following irradiation of two short but separated portions of the spinal cord. The material is evaluated qualitatively and quantitatively at the light microscopic level using both histopathologic and autoradiographic techniques. The findings are considered not only within the context of the immature nervous system but also of the mature nervous system. Perhaps this latter aspect will provide some insight into the reactions and the limitations characteristic of the mature state.